The prior art discloses several defect detection devices and processes using video cameras to produce a video signal which is then analyzed to detect a defect in the objects viewed by the cameras. U.S. patent application Ser. No. 07/493,011 filed Mar. 13, 1990, for WEB INSPECTION SYSTEM by Jean-Louis C. Guay (International Application Publication No. WO 91/14173 published Sep. 19, 1991) discloses the use of a plurality of cameras, each containing a TDI CCD array sensor, arranged in a line across a moving web for generating a plurality of continuous video signals representing images of respective portions of the moving web. These video signals are transmitted from the cameras to respective processing units which analyze the video signals for defects. The analog outputs of the TDI CCD array sensors are converted to streams of binary digital pixel signals either in the camera unit or in the processing units. The digital pixel signals are temporarily stored in computer memories of the processing units until they are analyzed by computers in the processing units to determine the presence of a defect.
One algorithm employed by the computers in the processing units of the above mentioned Guay application sequentially compares each digital pixel value of a line image across a web portion with a dynamically adjustable range. If a present pixel value is outside of the dynamic range but within predetermined ranges above or below the previously tested prior acceptable pixel value then the upper and lower limits of the dynamic range are changed up or down, respectively, by the difference between the present pixel value and the prior pixel value. This compensates for variations in thickness of the web, gradual increases and decreases in pixel values, which are normal and differentiate such normal changes from defects which create sudden changes greater than the predetermined ranges.
One deficiency of prior art defect detecting systems employing video cameras concerns the transmission of analog or digital signals from the cameras to the processing units and the temporary storage of these signals in the processing units. In web defect detection at relatively high resolution, required transmission rates can be up to or higher than thirty-two million bytes or pixels per second per camera. Reliable transmission and handling of a plurality of video signals at such frequencies is difficult and requires relatively expensive facilities. Large (several megabyte) high speed dual ported video memories with facilities to properly address and store incoming signals are required. These memories and their high speed computer processors are relatively complex and expensive. Even with such expensive high speed equipment, there are often unacceptable limitations on inspection rates and/or defect detection resolution.